Clicking noise after a few months parked

[Sarstan]

Parked my '91 318is for a few months, not started at all in that period.  Just charged the battery and started it up today to find it giving a clicking noise in relation to the RPM (revving makes it speed up).

Seems a lot of places are saying it's either low oil (oil level is fine, but parked at a slope), a valve is sticking (which perhaps that goes away with a bit of time?), or it's about to throw a rod.  All of these seem odd since the car was perfectly fine before being parked.

Should I keep running it for a bit to see if it goes away or is it time to pull out my tools?

[DesktopDave]

I'd think you have a stuck lifter.  It's a pretty mechanical tapping sound that varies precisely (at a 1:2 ratio) with RPM regardless of clutch position, road speed, gear, whatever....same tack-tack-tack.  It can be pretty loud, and it definitely seems to originate from the top of the motor.  The M42 was BMW's first engine with hydraulic lifters, they tend to bleed out after sitting a while.  Run the car, I think it'll go away.  If it doesn't, change to a slightly thinner oil, or add some synthetic oil to top it off.

A rod that's loose will sound like a clunking or clacking instead.  Definitely run the car.  They don't like to sit for too long.